This invention relates generally to methods and apparatus for eroding or ablating surfaces by lasers. In particular, this invention relates to methods and apparatus for ablating surfaces astigmatically, and for the surgical correction of myopic or hyperopic astigmatism through laser keratoplasty, keratomileusis or photorefractive keratectomy (PRK).
It is known to employ laser sources to erode surfaces of workpieces and the like. Such apparatus is in general relatively complex and demands highly skilled use. It is an object of the present invention to provide improved and simplified apparatus and method for eroding surfaces, particularly aspherical surfaces where it is typically desirable to reshape the surface into a more spherical shape having a symmetric and usually uniform curvature.
It is also an object of the present invention to provide an improvement whereby laser techniques can be applied to sensitive surfaces and, in particular, to objects in which it would be undesirable to affect underlying layers. This is particularly important in corneal surgery to correct vision defects, where upper layers of the cornea are sculpted to achieve the proper refractive power. Extreme care must be taken to avoid damage to the basement membrane and the posterior endothelial lining of the cornea in such operations.
In the field of medicine, a known technique for the treatment of certain forms of myopia is surgically to remove a segment of the collagen sub-surface layer of the eye, to reshape the removed segment as by surgical grinding, and to restore the reshaped segment in the eye. The eye heals by reformation of the outer cellular layer over the reshaped collagen layer. Alternatively, a layer of the cornea is opened up as a flap, an artificial or donor lenticular implant is inserted under the flap, and the flap is sutured up again.
It is a further object of this invention to provide an improved and less traumatic method and apparatus for reshaping the cornea of the eye.
Various other surgical techniques for reprofiling of the corneal surface have also been proposed. One increasingly common technique is radial keratotomy in which a set of radial incisions, i.e., resembling the spokes of a wheel, are made in the eye to remedy refractive errors, such as myopia (nearsightedness). As the incisions heal, the curvature of the eye is flattened, thereby increasing the ocular focal distance. The operation is not particularly suitable for correction of astigmatic (non-spherical) conditions and can pose problems, if the surgical incisions are uneven or too deep. Moreover, the results of such relaxing incisions are often hard to predict, especially in the correction of astigmatism.
The use of a laser beam as a surgical tool for cutting incisions, a so-called laser scalpel, has been known for some time (see, for example, U.S. Pat. No. 3,769,963 issued to Goldman et al.). In 1980, a study was made of the damage which might be inflicted on the corneal epithelium by exposure to the recently developed excimer laser (see Taboada et al., "Response of the Corneal Epithelium to ArF excimer laser pulses," Health Physics 1981, Volume 40, pp. 677-683). At that period, surgical operations on the cornea were commonly carried out using diamond or steel knives or razor, and further, such techniques were still being studied (see, for example, Binder et al., "Refractive Keratoplasty," Arch. Ophthalmol., May 1982, Vol. 100, p. 802). The use of a physical cutting tool in corneal operations and the insertion of an implant under a flap, continue to be widely practiced up to the present day (see, for example, "Refractive Keratoplasty improves with Polysulfone Pocket Incision," Ophthalmology Times, Jul. 1, 1986).
It has been suggested in U.S. Pat. No. 4,665,913 issued to L'Esperance that controlled, ablative, photo-decomposition of one or more selected regions of a cornea can be performed using a scanning action on the cornea with a beam from an excimer laser. Because of the scanning action, it is necessary for L'Esperance to bring his laser beam to a small spot, typically a rounded-square dot of size 0.5 mm by 0.5 mm.
L'Esperance suggests that myopic and hyperopic conditions can be reduced by altering the curvature of the outer surface of the cornea by repeatedly scanning the cornea with an excimer laser beam having this standard, small spot size but varying the field which is scanned during successive scans so that some areas of the cornea are scanned more often than others. In this way, it is claimed, the surface can be eroded by different amounts, depending on the number of times the spot scans the surface. In theory, at least, such scanning systems could be used to replace the shape of an astigmatic cornea with a more spherical shape.
In practice, however, complex apparatus is required to cause a pulsed laser beam to scan with the precision required, if the eroded surface is to be smooth. Thus, in successive sweeps of a scan overlap, there will be excessive erosion in the overlap area, whereas, if they fail to meet, a ridge will be left between the sweeps. The pulsed nature of excimer laser radiation also tends to exacerbate this problem. Additionally, the scanning method is inherently time-consuming even with highly refined techniques and apparatus, since the laser beam is only eroding a very small part of the total area to be treated at any given moment. Furthermore, such a scanning system can cause rippling effects on relatively soft materials, such as corneal tissue.
Another technique for corneal reshaping involves the use of a laser photoablation apparatus in which the size of the area on the surface to which the pulses of laser energy are applied is varied to control the reprofiling operation. In one preferred embodiment, a beam-shaping stop or window is moved axially along the beam to increase or decrease the region of cornea on which the laser radiation is incident. By progressively varying the size of the exposed region, a desired photoablation profile is established in the surface. For further details on this technique, see also Marshall et al., "Photo-ablative Reprofiling of the Cornea Using an Excimer Laser: Photorefractive Keratectomy," Vol. 1, Lasers in Ophthalmology, pp. 21-48 (1986), and U.S. Pat. No. 4,941,093 issued to Marshall et al., both of which are herein incorporated by reference.
Although this technique for varying the size of the exposed region is a substantial improvement over physical shaping (i.e., scalpel) techniques and laser spot scanning protocols, correction of astigmatic conditions is difficult. An astigmatic surface is typically defined by two natural and orthogonal curvatures which form the surface. A varying circular spot size will create or remove a single power (e.g., flatten or steepen the curvature relative to a point, the center of the optical axis).
To correct astigmatism, U.S. Pat. No. 4,941,093 teaches the use of specially configured optical elements or slits to provide ablation in one axis, i.e., such that the erosion proceeds selectively relative to a line rather than around a point. Typically, this approach requires a second step to provide spherical correction so that the proper overall curvature is achieved. In this approach, proper selection and precise superposition of two ablation patterns is necessary.
Yet another approach involves the use of a graded intensity laser beam or a graded absorption/transmission mask or a photodecomposable mask which varies the transmission of ablative laser radiation to the target surface, thereby inducing variable ablative depths on the surface. For example, U.S. Pat. No. 4,856,513 entitled "Laser Reprofiling Systems And Methods" which describes methodology for selectively eroding the cornea through the use of an erodable mask. The mask absorbs the surface laser radiation in varying amounts across the corneal surface to provide the desired surface profiles. This technique though requires the manufacture of a complementary object, i.e., the erodable mask, which compensates for the astigmatism and also requires precise correlative alignment with the axes of the target surface.
It is, accordingly, an object of this invention to provide a simpler method and apparatus for astigmatically reprofiling a surface with an initial, bi-powered, astigmatic shape in order to achieve a new, preferably spherical, shape.
It is another object of this invention to provide a method and apparatus for orienting and adjusting the astigmatic ratio applied to a surface shape with increased control.
It is further an object of this invention to provide a method and apparatus for correcting myopic astigmatism through corneal ablation in laser keratoplasty, keratomileusis or other PRK-type procedures.
These and other objects of the invention are evident in the description that follows.